The present invention relates in general to the art of diffusion bonding and, more particularly, to diffusion bonding of the superalloys utilizing a transient liquid phase in the bonding process.
Transient liquid phase diffusion bonding has been shown to be a very useful method for producing high quality diffusion bonds in the high temperature superalloys, such as those utilized in the manufacture of gas turbine engine hardware. Such bonding is described in detail in the U.S. Pat. No. 3,678,570 to D. F. Paulonis et al, of common assignee herewith, and whose teachings are incorporated herein by reference.
The superalloys are recognized as those alloys, usually having their basis in nickel, cobalt or iron, or some combination thereof exhibiting good high temperature strength and oxidation resistance in environments such as gas turbine engines. Usually, these alloys also contain substantial quantities of chromium and other elements such as aluminum, titanium and the refractory metals.
It is frequently desirable to make certain gas turbine engine components by joining easily fabricable segments together into the desired configurations. However, the limited weldability of many of these superalloys has severely limited the applicability of conventional joining techniques, such as fusion welding, in the production of structural hardware. Further, many components because of their design are simply not adapted to the utilization of fusion welding. Brazing, while offering a number of advantages over fusion welding, has very limited application because of the penalties associated with the relatively low strengths and low melting points of typical brazed joints.
The relative simplicity and reproducibility of the transient liquid phase diffusion bonding technique in the production of high quality bonds in sensitive hardware has led to substantial usage thereof. This is particularly true in the gas turbine engine industry although the invention described herein is obviously not limited thereto.
One key element in the transient liquid phase diffusion bonding technique is the provision between the surfaces to be joined of a thin alloy interlayer. This interlayer melts at a temperature below the melting temperature of the materials being joined, and through diffusion, solidifies at the joining temperature to form a bond. The composition of the interlayer preferably should be tailored to the alloys being joined, particularly with respect to the inclusion therein of those elements whose presence is required in the finished bond area and whose solid state diffusion rates are slow. It is also desirable to exclude from the interlayer alloy those elements which may adversely affect the bonding process or the quality of the finished joint. A melting point depressant (usually boron) is added to reduce the melting point of the interlayer to the desired point.
Since the amount of melting point depressant (boron) added to an interlayer to allow it to sufficiently melt at the bonding temperature also renders it extremely brittle; and therefore, unrollable in homogeneous form, other methods of applying such interlayers have been devised.
One method of getting the interlayer alloy species between the faying surfaces is through the use of a thin ductile foil of the type described in the U.S. Pat. No. 3,753,794 to D. F. Paulonis et al, which also shares a common assignee herewith. Ductile interlayer foil as described in this patent is manufactured by rolling the interlayer composition (minus the boron) into foil of the desired thickness and subsequently adding boron to the surfaces of the foil through a boronizing process. Another method is by providing an interlayer composition as a plate or coating (containing boron) on one or more of the faying surfaces themselves, as described in application Ser. No. 642,626 filed concurrently herewith. As previously mentioned it is generally important that at the completion of the diffusion bonding operation that the composition across the bond area include those elements whose presence is advantageous for optimum strength. In the case of some of the elements whose solid state diffusion rates are slow, it is desirable to provide these elements between the surfaces to be joined so that sole reliance on diffusion from the parent metal is not necessary. This is particularly true in the case of bonding gaps of substantial width because of the extended bonding cycle which would be required to provide diffusion over the greater distance.
In some cases, it is not feasible to provide all of the desired components in a single homogeneous interlayer foil (prior to boronizing) for several reasons. Principally, such complex alloys would either be physically impossible to roll into the thin foil required or would not melt sufficiently at the bonding temperature.